Aerial view of the
Pen Branch corridor, 1970's.
Steam rising around
dead cypress trees, Fourmile Creek, 1960's.
For over thirty years the bottomland hardwood forests of the Pen
Branch and Fourmile Creek corridors and deltas were subjected to the
discharge of coolant water from nuclear production reactors on the
Savannah River Site. Coolant waters, which reached flow rates of up
to 400 ft3/sec and temperatures of 40-50oC,
killed virtually all vegetation and eliminated the seed bank and root
stock from these bottomland hardwood wetlands. Release of heated effluents
ended in the mid-1980s. By the early 1990s, dense black
willow thickets covered the corridor of Pen Branch and, to a lesser
extent, Fourmile Creek, and there was very little evidence in either
of these stream systems of the pre-disturbance native bottomland hardwood
vegetation. Restoration efforts in these streams began with a research
study undertaken by the Savannah River Ecology Laboratory (SREL) in
Fourmile Creek. This study sought to determine optimal planting methods
and tree species to use when restoring native hardwood vegetation
to thermally impacted SRS stream corridors. In these studies, different
types and species of tree stock were transplanted to study areas in
the Fourmile Creek delta to examine effects of abiotic factors such
as fertilization and elevation and biotic factors such as competition
and herbivory. The results of SRELs research were used in 1992
when the U.S. Forest Service (USFS), in collaboration with the Savannah
River Technology Center (SRTC), SREL, and scientists from several
universities, began efforts to accelerate the restoration of the Pen
Branch stream system to its previous bottomland hardwood state. In
this effort, which was undertaken as a result of regulatory compliance
issues, approximately 85 ha over a 2.5-km section of the Pen Branch
corridor were planted with trees using site preparation techniques
that included no preparation, herbicides with burning, or herbicides
alone, with planted areas separated by strips of non-planted control
areas. Plantings included various species of oaks, hickories, persimmon,
green ash, sycamore, swamp tupelo, and bald cypress. Following each
planting, surveys were conducted to monitor tree survival and growth.
|SREL researcher sampling
invertebrates in Pen Branch.
Currently, the vegetation in Pen Branch is dominated by early successional
herbaceous species in planted areas and by a shrub canopy of black
willow and an understory of herbaceous species in control areas; in-stream
vegetation is dominated by dense beds of macrophytes. The open conditions
created by disturbance and site preparation have been conducive to
the establishment of early successional species. Based upon the elevated
fish populations present, it is apparent that insect populations in
Pen Branch are higher than in late successional stream systems. Amphibian
and reptile populations are well established and the overall abundance
of birds does not differ from late successional systems, although
the richness and diversity of bird species are not as great in the
Pen Branch corridor as in late successional stream systems.
|USFS/SRI personnel performing
a pre-planting burn in Pen Branch.
In conjunction with the vegetation planting undertaken in Pen Branch
by the USFS, a variety of research studies were begun to chart progress
toward recovery of this stream system. Research studies included investigations
of stream structure and function, surveys of the macroinvertebrates,
fish, amphibians, reptiles, birds, and mammals characteristic of Pen
Branch and control streams on the SRS, factors related to the survival
and growth of planted vegetation, characterization of soil nutrients
and carbon cycling in Pen Branch, and comparisons between impacted
and control streams to determine what aspects of the thermally impacted
streams are impaired and what factors may be operating to cause these
impairments to persist. Several studies in Pen Branch provided information
on how best to perform restoration in disturbed bottomland hardwood
systems. Specifically, studies determined that the highest probability
for tree seedling survival existed where shrub cover or a nurse crop
was present, protecting seedlings from herbivory by deer and feral
hogs. Other studies focused on abiotic components of wetland function,
such as soil and hydrologic conditions, nutrient turnover, light intensity,
and water chemistry. For restoration of Pen Branch to be successful,
these components must be restored to predisturbance conditions, or
at least be on a trajectory where restoration of such components is
SREL studies are comparing fish communities, riparian vegetation,
physical condition of the streams, and interactions among stream biota
in Pen Branch and Fourmile Creeks with control streams such as Upper
Three Runs and Meyers Branch. Seasonal fish community samples and
habitat information are being collected from 55 sites in these four
stream systems to document the recovery of streams from thermal disturbance
and to determine the factors important in successful recovery.
|Trees grown and planted
in Fourmile Creek as part of an SREL research program.
Results from SREL research in Fourmile
- Survival of planted vegetation was not enhanced by fertilization,
elimination of existing herbaceous vegetation, or removal of low
density black willow overstory.
- In areas of stream deltas where flooding by the Savannah River
is influential, species with the greatest flood tolerance, such
as bald cypress, water tupelo, green ash, and water hickory, are
required for successful tree establishment; protection from beaver
herbivory is critical.
- In areas of stream deltas where flooding is not influential, tree
species with less flood tolerance, including Nuttall, overcup, water,
willow, and swamp chestnut oaks, should be used to increase diversity;
beaver herbivory is not a general concern, although deer and feral
hogs can cause local problems.
Results from SREL research in Pen Branch...
- Fish densities are 2-8 times higher in disturbed streams than
in control systems.
- Fish species tolerant of conditions associated with more open
canopies of willows and shrubs, such as minnows, sunfish, suckers,
and mosquitofish, comprise 80-90% of the individuals in disturbed
streams but only 40-50% in control streams.
- Increased aquatic macrophytes that resulted from canopy alterations
have decreased the stability of the stream bottom topography by
trapping sediments and diverting water flows, causing adjacent scouring.
Topography of the stream bottom changes as macrophyte beds grow,
die off, or change in shape.
- Differences in types and abundance of macroinvertebrates were
observed between the experimentally planted thermally impacted regions
of Pen Branch and upper reaches of the stream that were not subjected
to discharge of thermal effluents.
- Significant differences, including hydrology, still exist between
Pen Branch and unimpacted control streams such as Meyers Branch.
- Short-term effects of restoration activities included increased
amounts of small woody debris, produced when herbicide-sprayed willows
and shrubs died and fell into the water, further increased aquatic
macrophyte growth due to the removal of the willow canopy, and additionally
increased fish abundances. However, it is becoming clear that the
hardwood forest will need to be reestablished if thermally impacted
streams are ever to exhibit levels of ecological integrity and diversity
similar to undisturbed control streams. Additional time and research
will be required to assess the long-term effectiveness of current
Thermally Impacted Streams
to Research Snapshots)